Ceramic adhesive

ABSTRACT

A ceramic adhesive comprises a ceramic matrix formed from a mixture of a metallic oxide and an alkali silicate in water, and silicon carbide whiskers dispersed within the ceramic matrix. Further, a ceramic adhesive comprises a ceramic matrix formed from a mixture of magnesium oxide in the range of about 50% to about 80% by weight and sodium silicate in water in the range of about 50% to about 20% by weight, and silicon carbide whiskers dispersed within said ceramic matrix, the whiskers being in the range of about 3% to about 25% by weight of the ceramic matrix. A ceramic adhesive of the present invention is particularly useful in mounting and bonding in-cylinder sensors to steel internal engine parts.

TECHNICAL FIELD

The present invention relates to a ceramic adhesive, and moreparticularly to a high temperature, abrasion resistant ceramic adhesivereinforced with silicon carbide whiskers.

BACKGROUND ART

Ceramic adhesives have properties such as good mechanical strength andhigh temperature stability, making them suitable for high technologyapplications. However, higher porosity and lower fracture toughness thandesirable are some of the limitations of ceramic adhesives. Hence,recent work in this field has been directed to the development oftougher and more reliable ceramic adhesive materials.

In the past, ceramic adhesives have been used in various hightemperature applications. However, most of these applications have beenof a nature where the ceramic adhesive has been subjected mainly tothermal stresses, and not a combination of thermal and mechanicalstresses, chemically corrosive environment and abrasive wear. U.S. Pat.No. 5,006,423 issued Apr. 9, 1991 to Draskovich, discloses a refractorycement formed from a mixture of sodium silicate in water and a ceramicpowder, such as silicon nitride, silicon carbide or silicon dioxide.Draskovich provides this refractory cement for bonding metallicinstrumentation to ceramic components of a gas turbine engine, for useat temperatures below about 2500° F. Draskovich does not solve theproblem of bonding metallic instrumentation to metallic parts, where theceramic adhesive must be durable and long-lasting, in spite of thedifferences in thermal coefficient of expansion and thermalconductivity, between the metallic instrumentation, the ceramic adhesiveand the metallic component.

It has been known that ceramic composites can be reinforced by usingfibers. It is also known that silicon carbide fibers toughen thematerial through the mechanisms of crack deflection and fiber pullout.However, silicon carbide fibers tend to soften at temperatures of about850° C. and lose their toughening effect. Several inventors havediscovered the advantages of using silicon carbide whiskers (SiCwhiskers) in ceramic compositions. U.S. Pat. No. 5,108,963 issued Apr.28, 1992 to Fu et. al, discloses a SiC whiskers reinforced alumina basedceramic composite which shows improved mechanical properties andsinterability through the addition of chromia. However, this ceramicdoes not have any adhesive properties. Further, it requires a sinteringtemperature of about 1350° C. Such a high temperature destroys thesensor mounted in the engine part.

U.S. Pat. No. 4,231,800 issued Nov. 4, 1980 to Holt et. al, discloses adry heat setting refractory for securing nozzles to nozzle blocks insteel pour ladles. Holt discloses a refractory composition comprisingmagnesite and hydrated sodium silicate in a weight ratio in the range of94:6 to 98:2. Holt preferably uses hydrated sodium silicate so that therefractory material is dry and a bond is formed when hot steel engagesthe dry refractory. Although the type of ceramic adhesive disclosed byHolt may be suitable for bonding nozzles to nozzle blocks in steel pourladles, it has been found to be not useful in mounting and bondinginstrumentation on components inside an internal combustion engine. Forexample, an adhesive used for mounting a motion sensor in a piston-ringgroove, is subjected to temperatures in the range of about 500°-600° C.,combustion gases and lubricant flow. All of these work together tocreate a tribological wear environment that eventually erodes theadhesive and destroys the bond between the instrumentation and theengine component.

In the present invention, it has been found that a ceramic adhesiveformed from a mixture of an alkali silicate in water and magnesiumoxide, can be substantially toughened by the addition of silicon carbide(SIC) whiskers. Further, because this ceramic adhesive is cured at lowtemperatures, it is believed that there is no chemical bonding of thewhiskers with the ceramic matrix. Hence, the desirable propertiesimparted by the SiC whiskers, like impeding crack propagation throughmechanisms such as crack bridging, crack deflection and whisker pullout,are exploited to impart a combination of good mechanical, thermal,chemical and abrasion resistance properties to the ceramic adhesive.

It is also known to bond sensors to components by means of otheradhesives, such as organic epoxy glues. Epoxy glues have good bondingcharacteristics, but they are limited to temperatures below about 260°C. It has been also discovered that an epoxy can be used to seal theporosity of a ceramic adhesive, resulting in an overall improvement inthe mechanical, thermal, chemical and abrasion resistance properties ofthe sealed ceramic adhesive.

It is desirable to have a tough ceramic adhesive for bondinginstrumentation such as ring motion sensors, pressure sensors,thermocouples, and liner gap sensors, on engine components such ascylinder walls and pistons, that can withstand thermal and mechanicalstresses, and the deleterious effect of combustion gases, enginelubricants and abrasive wear. The present invention is directed toovercome one or more of the problems as set forth above.

DISCLOSURE OF THE INVENTION

In one aspect of the invention, a ceramic adhesive comprises a ceramicmatrix formed from a mixture of a metallic oxide and an alkali silicatein water, and silicon carbide whiskers dispersed within the ceramicmatrix.

In another aspect of the invention, a ceramic adhesive comprises aceramic matrix formed from a mixture of magnesium oxide in the range ofabout 50% to about 80% by weight and sodium silicate in water in therange of about 50% to about 20% by weight, and silicon carbide whiskersdispersed within said ceramic matrix, the whiskers being in the range ofabout 3% to about 25% by weight of the ceramic matrix.

A ceramic adhesive of the present invention is particularly useful inmounting and bonding in-cylinder sensors to steel internal engine parts.

BEST MODE FOR CARRYING OUT THE INVENTION

In the preferred embodiment of the present invention, the metallic oxideused is magnesium oxide. The bulk density of magnesium oxide powder isdesirably in the range of 1.90 gms/cc and 1.99 gms/cc, and preferably,about 1.94 gms/cc. The particle size of the magnesium oxide powder isdesirably within the range of 0.1 μm to 20 μm, and preferably within therange of 0.3 μm to 3.0 μm. Even more preferably, at least 70% of themagnesium oxide powder should have a particle size of about 0.7 μm andthe balance within the range of about 0.3 μm to 0.7 μm. A homogeneouslyuniform particle size of MgO is useful in attaining a densemicrostructure and a grain size within the range of about 0.1 μm to 1.0μm in the ceramic adhesive matrix. It has been found that a fine grainmicrostructure densifies the adhesive, reduces its porosity and improvesthe bond between the sensor and the engine component.

Other examples of metallic oxides that may alternatively be used arealuminum oxide (Al₂ O₃), silicon dioxide (SiO₂), zirconium oxide (ZrO₂)and other suitable equivalents. However, magnesium oxide (MgO) ispreferred because it has been found that by using MgO, the resultantceramic adhesive has a coefficient of thermal expansion very close tothat of steel, i.e., about 10 μm/m.°K. It is desirable that the ceramicadhesive have a coefficient of thermal expansion in the range of about10 μm/m.°K. to 14 μm/m.°K., and preferably, about 12 μm/m.°K., tominimize the thermal stresses caused by the thermal expansion andcontraction of the steel engine component, the sensor, and the ceramicadhesive. Hence, Al₂ O₃, SiO₂ and ZrO₂ may be used as alternatives toMgO but it should be noted that magnesia has a higher thermalconductivity than alumina, silica and zirconia and thus, the choice ofan alternative should be based on the desired coefficient of thermalexpansion of the ceramic adhesive and coefficient of thermal expansionof the metal that the engine component is made from. The magnesium oxidepowder used for carrying out an embodiment of the present invention hasa trade name "Ceramabond 571 powder", is manufactured by AremcoProducts, Inc., and has a particle size distribution such that about 30%of the powder has a particle size of about 20 μm, and about 70% of thepowder has a particle size within the range of about 1 μm to 10 μm.

The term "alkali silicate in water", as used herein means a compoundhaving a formula: (Na₂ O)_(x).(SiO₂)_(y).(H₂ O)_(z) or (K₂O)_(x).(SiO₂)_(y).(H₂ O)_(z), where the ratio of y:x is in the range ofabout 2.8:1 to about 3.6:1 by weight and the ratio of z:(x+y) is in therange of about 0.3:1.0 to 0.5:1.0 by weight.

In the preferred embodiment of the present invention, the alkalisilicate used is sodium silicate in water, i.e., water glass, having theformula (Na₂ O)_(x).(SiO₂)_(y).(H₂ O)_(z). The water content of sodiumsilicate in water is desirably within the range of about 30% to 50%, andpreferably, about 40% by weight. The ratio of silica to soda isdesirably within the range of about 2.8:1 to 3.6:1, and preferablywithin the range of 3.1:1 to 3.3:1. The viscosity of sodium silicate inwater is desirably within the range of about 300 centipoise to 800centipoise, and preferably about 550 centipoise. The specific gravity ofsodium silicate in water is desirably within the range of about 1.37 to1.42, and preferably, about 1.39. Another example of a suitablealternative alkali silicate is potassium silicate. The sodium silicateused for carrying out an embodiment of the present invention has a tradename "Ceramabond 571 liquid", is manufactured by Aremco Products, Inc.,and has a viscosity of about 500 centipoise, a specific gravity of about1.395 and a water content of about 35%.

In the preferred embodiment of the present invention, the length of thesilicon carbide whiskers is desirably within the range of about 10 μm to150 μm, and preferably, about 125 μm. The diameter of the whiskers isdesirably within the range of about 0.1 μm to 1.0 μm, and preferably,about 0.3 μm. The length to diameter ratio is desirably within the rangeof about 200 to 600, and preferably, about 416. It has been discoveredthat the dispersion of SiC whiskers within the adhesive matrix impartstoughness to the adhesive and significantly improves the durability ofthe adhesive, when exposed to the severe environment inside a combustionengine.

In the below described illustrative Examples A, B and C, sample 1, aceramic adhesive having SiC whiskers, and used for carrying out anembodiment of the present invention, was formed in the following manner:Ceramabond 571 powder (magnesium oxide) and SiC whiskers were mixed in aball mill to form a powder mix, such that the whiskers comprised about15% by weight of said powder mix. To 2 parts by weight of said powdermix, 1 part by weight of Ceramabond 571 liquid (sodium silicate inwater) was added, resulting in a viscous slurry of a wet adhesivemixture having the composition by weight %:

    ______________________________________                                        magnesium oxide     56.67                                                     sodium silicate in water                                                                          33.33                                                     silicon carbide whiskers                                                                          10.00                                                     ______________________________________                                    

In the below described illustrative Examples A, B and C, sample 2, aceramic adhesive without any SiC whiskers, was formed in the followingmanner: 2 parts by weight of Ceramabond 571 powder (magnesium oxide)were mixed with 1 part by weight of Ceramabond 571 liquid (sodiumsilicate in water), resulting in a viscous slurry of a wet adhesivemixture having the composition by weight %:

    ______________________________________                                        magnesium oxide     66.67                                                     sodium silicate in water                                                                          33.33                                                     ______________________________________                                    

In the below described illustrative Examples A, B and C, the ceramicadhesives of samples 1 and 2 were both cured in the following manner:the wet ceramic adhesive slurry was dried by placing inside an oven atabout 50° C. and increasing the temperature of the oven from 50° C. to94° C. over a period of about 24 hours. Thereafter, the ceramic adhesivewas cured by increasing the temperature from 94° C. to 105° C. over aperiod of 16 hours, and further increasing the temperature from 105° C.to 260° C. over a period of about 8 hours. The temperature was heldconstant at about 260° C. for about 2 hours and after that, the curedadhesive was allowed to cool down to room temperature. It is of extremeimportance that in the curing cycle, the temperature is gradually raisedas described above, or else bubbling of the adhesive will occur.

For all the test samples in the below described illustrative Examples,the coefficient of thermal expansion was measured by a Theta PushrodRecording Dilatometer in a temperature range of from room temperature toabout 1000° C. and the porosity was measured by a Mercury Porosimeter.These techniques are well known to a person having an ordinary skill inthe art.

EXAMPLE A

Samples 1 and 2 were formed and cured in the manner described above.Several specimens of both samples were formed in the shape of bars,having a length of about 4 inches and a diameter of about 1". Aftercuring, both adhesive samples were heat aged for a period of about 8hours in an oven heated to a temperature of about 343° C. After thisheat aging, both samples were removed from the oven and examined for anyappearance of any spontaneous brittle fracture. Then, both adhesivesamples were struck with a 5 lb. hammer to examine whether the adhesivecracked. The following observations were made:

    ______________________________________                                        Sample 1     No spontaneous cracks                                                         Sample did not break                                             Sample 2     Several spontaneous cracks                                                    Sample broke and became powdery                                  ______________________________________                                    

It can be seen that sample 1, which had SiC whiskers, had substantiallybetter fracture toughness after heat ageing than sample 2, which had noSiC whiskers.

EXAMPLE B

The ceramic adhesives of sample 1 and sample 2 were tested in a dieselengine environment. A ring motion sensor was installed in the top pistonring groove of a steel piston. This was done by drilling bores having adiameter of about 2 mm, in the piston ring groove, said bores defining acircular passage extending from the circumference of the piston ringgroove to the back side (the oil gallery side) of the piston. The sensorand the sensor wires were encapsulated in the ceramic adhesives ofsample 1 and 2 and positioned inside the bores. The adhesives were curedas described above and the entire assembly, comprising the piston andthe ring motion sensor and wire, bonded in place by the adhesives ofsamples 1 and 2, was installed in a diesel engine. The engine was run ona test bench for a period of about 10 hours. The sensors was examined atthe end of the run and the following observations were made:

    ______________________________________                                        Sample 1      Sensor somewhat loose, adhesive                                               looks good                                                      Sample 2      Sensor loose, adhesive appears                                                cracked and brittle                                             ______________________________________                                    

EXAMPLE C

The coefficient of thermal expansion, specific gravity and porosity ofsamples 1 and 2 were measured and found to be as follows:

    ______________________________________                                                           Sample 1                                                                             Sample 2                                            ______________________________________                                        Coeff. of thermal exp., μm/m. °K.                                                        10       10                                              Specific gravity     2.11     2.34                                            % Porosity           33       26                                              Avg. pore diameter, μm                                                                          0.0726   0.188                                           Median pore diameter, μm                                                                        0.0047   0.0044                                          ______________________________________                                    

It was observed that the bond integrity of the sensor bonded with sample1 was better than sample 2 in an actual engine environment. It isbelieved that sample 1 performed better due to its higher thermalconductivity as compared to sample 2. Because sample 1 has SiC whiskersdispersed in the ceramic matrix and because the SiC whiskers have athermal conductivity about twice that of the ceramic matrix, it isbelieved that sample 1 has a higher thermal conductivity than sample 2.The coefficient of thermal expansion of steel is about 12 μm/m.°K.,while that of sample 1 and sample 2 is about 10 μm/m.°K. It is believedthat the SiC whiskers improve the fracture toughness of the ceramicadhesive. This is because although the porosity of sample 1 is somewhathigher than sample 2, the average pore diameter of sample 1 is less than40% of that of sample 2. Thus sample 1 has a more uniform poremicrostructure and a finer grain size.

In summary, the ceramic adhesive embodying the present invention hasimproved thermal conductivity, coefficient of thermal expansion andfracture toughness over conventional ceramic adhesives. This results ina ceramic adhesive having thermal properties similar to steel. Thisreduces thermal stresses due to differences in expansion rates andtemperature gradients.

Industrial Applicability

A ceramic adhesive of the present invention is particularly useful inmounting and bonding in-cylinder sensors to steel internal engine parts.Such sensors are typically used to measure temperature, pressure,acceleration, piston ring motion and piston ring to cylinder liner gap,in engines being tested during research and development. The ceramicadhesive described herein is especially useful in encapsulating a sensorand retaining it within a small bore in a piston ring groove bymaintaining a firm bond between the sensor and the steel substrate.

Further, the ceramic adhesive described herein has good resistance tofracture and abrasion and has a thermal conductivity and coefficient ofthermal expansion similar to steel, resulting in lower thermal stresses.Hence, the sensors mounted by using this ceramic adhesive will stay inplace longer.

Other aspects, objects and advantages of this invention can be obtainedfrom a study of the disclosure and the appended claims.

I claim:
 1. A ceramic adhesive, comprising:a ceramic matrix, saidceramic matrix being formed from a mixture of a metallic oxide and analkali silicate in water; and silicon carbide whiskers dispersed withinsaid ceramic matrix; wherein said ceramic matrix is formed from amixture, comprising, in the range of about 50% to about 80% by weightmetallic oxide and in the range of about 50% to about 20% by weightalkali silicate in water, and said silicon carbide whiskers beingpresent in the range of about 3% to about 25% by weight of said ceramicmatrix; and wherein said metallic oxide is selected from a groupconsisting of magnesium oxide, aluminum oxide, zirconium oxide, silicondioxide and mixtures thereof.
 2. A ceramic adhesive, as set forth inclaim 1, wherein said ceramic matrix is formed from a mixture,comprising, about 63% by weight metallic oxide and from about 37% byweight alkali silicate in water, and said silicon carbide whiskers beingpresent in about 10% by weight of said ceramic matrix.
 3. A ceramicadhesive, as set forth in claim 1, wherein said metallic oxide isselected from the group consisting of aluminum oxide, silicon dioxide,zirconium oxide and mixtures thereof.
 4. A ceramic adhesive, as setforth in claim 3, wherein said metallic oxide has a particle size in therange of about 0.1 μm to 20 μm.
 5. A ceramic adhesive, as set forth inclaim 1, wherein said alkali silicate in water is selected from thegroup consisting of sodium silicate in water and potassium silicate inwater and mixtures thereof.
 6. A ceramic adhesive as set forth in claim5, wherein said alkali silicate in water has a specific gravity in therange of about 1.37 to 1.42.
 7. A ceramic adhesive, as set forth inclaim 5, wherein said alkali silicate in water has a viscosity in therange of about 300 centipoise to 800 centipoise.
 8. A ceramic adhesive,as set forth in claim 5, wherein said alkali silicate in water is sodiumsilicate in water.
 9. A ceramic adhesive, as set forth in claim 8,wherein said sodium silicate in water has a specific gravity in therange of about 1.37 to 1.42 and a viscosity in the range of about 300centipoise to 800 centipoise.
 10. A ceramic adhesive, as set forth inclaim 8, wherein said sodium silicate in water has the formula (Na₂O)_(x).(SiO₂)_(y).(H₂ O)_(z), the ratio of y:x is in the range of about2.8:1 to about 3.6:1 by weight and the ratio of z:(x+y) is in the rangeof about 0.3:1.0 to 0.5:1.0 by weight.
 11. A ceramic adhesive, as setforth in claim 10, wherein the ratio of y:x is about 3.3:1 by weight andthe ratio of z:(x+y) is about 0.4:1 by weight.
 12. A ceramic adhesive,as set forth in claim 1, wherein said silicon carbide whiskers have alength in the range of about 10 μm to 150 μm, a diameter in the range ofabout 0.1 μm to 1.0 μm, and a length to diameter ratio in the range ofabout 200 to
 600. 13. A ceramic adhesive, as set forth in claim 12,wherein said silicon carbide whiskers have a length of about 125 μm, adiameter of about 0.3 μm, and a length to diameter ratio of about 416.14. A ceramic adhesive, as set forth in claim 1, Wherein said ceramicadhesive has a coefficient of thermal expansion in the range of about 9μm/m.°K to 14 μm/m.°K.
 15. A ceramic adhesive, as set forth in claim 1,wherein said ceramic adhesive has a porosity less than about 40%.
 16. Aceramic adhesive, as set forth in claim 1, wherein said ceramic adhesivehas an average pore diameter in the range of 0.05 μm to 0.10 μm.
 17. Aceramic adhesive, comprising:a ceramic matrix, said ceramic matrix beingformed from a mixture, comprising, in the range of about 50% to about80% by weight magnesium oxide and in the range of about 50% to about 20%by weight sodium silicate in water; and silicon carbide whiskers beingdispersed within said ceramic matrix, said whiskers being in the rangeof about 3% to about 25% by weight of said ceramic matrix.
 18. A ceramicadhesive, as set forth in claim 17, wherein said magnesium oxide has aparticle size in the range of about 0.1 μm to 20 μm.
 19. A ceramicadhesive, as set forth in claim 18, wherein said magnesium oxide has aparticle diameter in the range of about 0.3 μm to 3.0 μm.
 20. A ceramicadhesive, as set forth in claim 17, wherein said sodium silicate inwater has a specific gravity in the range of about 1.37 to 1.42 and aviscosity in the range of about 330 centipoise to 760 centipoise.
 21. Aceramic adhesive, as set forth in claim 17, wherein said silicon carbidewhiskers have a length in the range of about 10 μm to 150 μm, a diameterin the range of about 0.1 μm to 1.0 μm, and a length to diameter ratioin the range of about 200 to
 600. 22. A ceramic adhesive, as set forthin claim 17, wherein said ceramic adhesive has a porosity less thanabout 40%, and an average pore diameter in the range of 0.05 μm to 0.10μm.